interspaces of the main tubes. These tubes, at their origin, present
a diameter of 8ööö of an inch, with interspaces equal to those of their
diameters. The smallest branches into which they are resolved, and
which are lost in the clear enamel-like external stratum of the
tooth, have a definable diameter of ^ 7 of an inch. This external
layer is thickest at the upper surface of the tooth, and the boundary
between it and the tubular substance of the tooth, is here defined
by a stratum of calcigerous cells. Some of the fine peripheral
branches of the calcigerous tubes unite and form loops, the convexity
of which is turned towards the cells, and from which the finer
branches proceed to communicate with the superficial calcigerous
cells.
17. Myliobates.—The modification of the plagiostomous type of
teeth, for the purpose of crushing the alimentary substances, is most
complete in this genus. A view of this armature of the mouth, as
seen from behind in the Myliobates Aquila is given PI. 25, fig. 1. Both
jaws are covered with a pavement of broad teeth, with a flat grinding
surface, vertical and finely undulated sides, by which contiguous
teeth are joined together as by a suture, (PL 27, c) and a base
divided into a number of small parallel longitudinal ridges.
The entire phalanx of dental plates of the upper jaw describes
the segment of a circle. In the Myliobates marginata (suhg. Rhi-
noptera of Kuhl), the teeth are hexagonal, the middle row being the
broadest, and the lateral ones diminishing as they recede; the
outermost are mostly pentagonal. I have seen an example of the
jaws of this suhgenus of Myliobates, in which one of the rows next
the median was subdivided into two unequal series, as represented in
fig. 2, pi. 25. In Myl. Jussieui fZygobates, Agass,), the three middle
rows of teeth are much broader than the two marginal rows. In Myliobates
proper, the median row itself forms the principal part of the
dental covering of the jaws, which is bordered by three narrow rows of
teeth, having the same antero-posterior extent as the median plates.
Finally, in Myliobates Narinari fAetobatis, Müller), the small marginal
teeth have entirely disappeared, and the jaws support a single row of
broad dental plates. In the upper jaw, these plates are arched, with the
convexity turned forwards; in the lower jaw, they pass straight
across, with the extremities only a little bent backwards. The jaws
which support and work these dense and heavy teeth, are proportionally
strong ; and in Aetobatis, they nearly approach to the
density of true bone. In this subgenus, the upper jaw is shorter
and more curved than the lower one, the anterior extremity of which
projects beyond the upper jaw, and can be used, like a spade, in
digging out shell-fish, &c., from the sandy bottoms frequented by
these rays. (1)
Microscopic examination of the texture of the teeth of Myliobates
has yielded the following results. A longitudinal and vertical
section of a single dental plate, viewed by a compound lens of an
inch focus (PL 26), exhibits at its base (a) a coarse network of large
irregular canals, filled with a vascular medullary pulp. From
this network smaller medullary canals proceed, towards the flat
grinding surface, in a straight and slightly diverging course,
subdividing dichotomously with interspaces equal to their own
diameter at the base, but much wider at the working surface of the
tooth. In a transverse section of the tooth, under the same power,
the area of the medullary canals is seen to present generally an
irregular elliptical form, from which radiating calcigerous tubes are
faintly perceptible. Each canal and its series of tubes is surrounded
by a line of generally a hexagonal form, and which constitutes the
boundary between contiguous canals and tubes, the whole tooth being
thus composed of an aggregate of slender, elongated, commonly six-
sided prismatic teeth, placed vertically to the grinding surface. A
section through the roots of the tooth (b, pi. 26), shows that these
parts are occupied by a network of irregular canals, which anastomose
by arched branches with the network of the contiguous root,
and these with the network of coarser tubes which occupy the basis
of the tooth for an extent exceeding the length of the root itself.
With a higher power, |th inch focus, the calcigerous tubes are
seen to radiate in all directions from the medullary canals, and are
sent off throughout the whole course of the canal. These tubes are
short, wavy, richly arborescent, and form numerous anastomoses with
each other. The transverse sections of the tooth show that the area
of each medullary canal has been filled up or diminished by the depo-
(1)P1. 16, fig. 1 .